223Ra-dichloride spectrometric characterization: Searching for the presence of long-lived isotopes with radiological protection implications

Abstract

223Ra-dichloride was approved with the commercial name of Xofigo in 2014 for treatment of metastatic castration-resistant prostate cancer.223Ra is obtained by neutron irradiation of 226Ra yielding 227Ac, which decays to 227Th and 223Fr, both decaying to 223Ra. Since 223Ra is predominantly (95.3%) an alpha emitter with a 11.42days long half-life, the radiopharmaceutical, its remnants, the patient, and waste material can be managed and disposed with low radiation protection requirements.227Ac is a long-lived (T1/2=21.77years) beta emitter that demands strong radiation protection measures. In particular waste disposal has to follow the International Atomic Energy Agency (IAEA) and European Commission (EC) regulations. Since 227Ac is involved in the production of 223Ra, an impurity analysis of each batch is required after production. Due to time restrictions, the manufacturer’s detection limit (<0.001%) exceeds the one required to assure that 227Ac concentrations are below direct disposal levels. To improve the detection limit, long-term accurate spectroscopy is required.Alpha and gamma spectroscopy measurements were carried out at the Complutense University Nuclear Physics Laboratory. After twelve months follow up of a sample, 227Ac concentration was found to be smaller than 10−9. This allows for direct waste disposal and no additional radiation protection restrictions than those required for 223Ra. The presence of contamination by other radioisotopes was also ruled out by this experiment. Specifically 226Ra, involved in 223Ra production as the original parent and with a very long-lived (T1/2=1577years) alpha emitter, was also below the experimental detection limit.